Its appearance is a matter of debate, for no representative has
been (or is ever likely to be) identified in the fossil record; the reconstructed morphology it would display
largely depends on whether the acoelomorpha are included within the
bilaterian clade. Since all protostomes and deuterostomes share features, such as blood
circulation systems and guts, that are only useful in
relatively large (macroscopic) organisms, their common ancestor
ought also to have been macroscopic. However, such large animals
should have left traces in the sediment in which they
moved, and evidence of such traces first appear relatively late in
the fossil record — long after
the urbilaterian would have lived. This leads to suggestions of a
small urbilaterian, which is the supposed state of the ancestor of
protostomes, deuterostomes and acoelomorphs.

Contents

Dating
the urbilaterian

The first evidence of bilateria in the fossil record comes from
trace fossils in
sediments towards the end of the Ediacaran period (about 570 million years
ago), and the first fully accepted fossil of a bilaterian
organism is Kimberella, dating to 555 million years
ago.[1] There
are earlier, controversial fossils: Vernanimalcula has been interpreted
as a bilaterian, but may simply represent a fortuitously infilled
bubble.[2]
Interestingly, Fossil embryos are known from around the
time of Vernanimalcula (580 million years
ago), but none of these have bilaterian affinities.[3] This
may reflect a genuine absence of bilateria, but caution is due — it
could be that bilateria didn't lay eggs in sediment, where they
would be likely to fossilise.[4]

Molecular
techniques can generate expected dates of the divergence
between the bilaterian clades, and thus an assessment of when the
urbilaterian lived. These dates have huge margins of error, though
they are becoming more accurate with time. More recent estimates
are compatible with an Ediacaran bilaterian, although it is
possible, especially if early bilaterians were small, that the
bilateria had a long cryptic history before they left any evidence
in the fossil record.[5]

Characteristics of the
urbilaterian

The urbilaterian must have possessed all those traits common to
living bilateria that are thought only to have evolved once. The
defining features are a three-layered (triploblastic) embryo
and two main axes of symmetry — a top-to-bottom axis and a
front-to-back axis.[6]

A traditional approach to reconstructing the urbilaterian
considers it as a "roundish flatworm" consisting of as few features
as possible. This concept of a very simple animal was abandoned
when it was realised that the vast array of traits common to
bilaterians would produce quite a complex animal,[5]
but has had a resurgence with the inclusion of a phylum of
flatworms within the bilateria.[6]

The urbilaterian is often considered to have possessed a gut and
internal organs, a segmented body and a centralised
nervous
system, as well as a biphasic life cycle (i.e. consisting of
larvae and adults) and some features of embryonic development. However, this need
not necessarily be the case.[6]
If these features were of key importance to the evolution of large
size, it is possible that bilaterian lineages each stumbled upon
them independently, and perhaps even co-opted the same underlying
genetic machinery from a different role.[5]

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Eyes

Light detection (photosensitivity) is present in organisms as
simple as seaweeds; the definition of a true eye varies, but in
general eyes much have directional sensitivity, and thus have
screening pigments so only light from the target direction is
detected. Thus defined, they need not consist of more than one
photoreceptor cell.[7]

The presence of genetic machinery (the Pax6 and
Six genes) common to eye formation in all bilaterians
suggests that this machinery - and hence eyes - was present in the
urbilaterian.[7]
The most likely candidate eye type is the simple pigment-cup eye,
which is the most widespread among the bilateria.[7]

Since two types of opsin, the c-type and r-type, are found in
all bilaterians, the urbilaterian must have possessed both types -
although they may not have been found in a centralised eye, but
used to synchronise the body clock to daily or lunar variations in
lighting.[8]

Complex or
simple?

Proponents of a complex urbilaterian point to the shared
features and genetic machinery common to all bilateria. They argue
that (1) since these are similar in so many respects, they can only
have evolved once; and (2) since they are common to all bilateria,
they must have been present in the ancestral bilaterian animal.

However, as biologists' understanding of the major bilaterian
lineages increases, it is beginning to appear that some of these
features may have evolved independently in each lineage. Further,
the bilaterian clade has recently been expanded to include the
acoelomorphs — a group of relatively simple flatworms. This lineage
lacks key bilaterian features, and if it truly does reside within
the bilaterian "family", many of the features listed above are no
longer common to all bilateria.[6]
Instead, some features — such as segmentation and possession of a
heart — are restricted to a sub-set of the bilateria, the
deuterostomes and protostomes. Their last common ancestor would
still have to be large and complex, but the bilaterian ancestor
could be much simpler.[6]
However, some scientists stop short of including the acoelomorph
clade in the bilateria. This shifts the position of the cladistic node which is
being discussed; consequently the urbilaterian in this context is
farther out the evolutionary tree
and is more derived than the
common ancestor of deuterostomes, protostomes and
acoelomorphs.[9]

Genetic reconstructions are unfortunately not much help. They
work by considering the genes common to all bilateria, but
problems arise because very similar genes can be co-opted for
different roles. For instance, the gene Pax6 has a key role in eye
development, but is absent in some animals with eyes; some cnidaria have genes which in
bilateria control the development of a layer of cells that the
cnidaria don't even possess. This means that even if a gene can be
identified as present in the urbilaterian, we can't tell what the
gene was coding for.[6]
Before this was realised, genetic reconstructions implied a
surprisingly complex urbilateria.[5]

Reconstructing the
urbilaterian

The absence of a fossil record gives a starting point for the
reconstruction — the urbilaterian must have been small enough not
to leave any traces as it moved over or lived in the sediment
surface. This means it must have been well below a centimetre in
length. As all Cambrian animals are marine, it is reasonable to
assume that the urbilaterian was too.[6]

Furthermore, a reconstruction of the urbilateria must rest on
identifying morphological similarities between all bilateria. While
some bilateria live attached to a substrate, this appears to be a
secondary adaptation, and the urbilaterian was probably mobile.[6]
Its nervous
system was probably dispersed, but with a small central
"brain". Since acoelomorphs lack a heart, coelom or organs, the
urbilaterian probably did too — it would presumably have been small
enough for diffusion to
do the job of transporting compounds through the body.[6]
A small, narrow gut was probably present, which would only have had
one opening — a combined mouth and anus.[6]
Functional considerations suggest that the surface of the
bilaterian was probably covered with cilia, which it could have used for
locomotion or feeding.[6]

There is still no consensus on whether the characteristics of
the deuterostomes and protostomes evolved once or many times.
Features such as a heart and blood circulation system may therefore
not have been present even in the deuterostome-protostome ancestor,
which would mean that this too could have been small (hence
explaining the lack of fossil record).[5]